select [distinct] 字段1 [as 别名], ..., 字段n [as 别名] from [库名.]表名[where 约束条件group by 分组依据having 过滤条件order by 排序的字段limit 限制显示的条数];
注：
1.查表中所有字段用*表示(select * from ...)
2.条件的书写规则严格按照语法顺序书写，可以缺省，但不可以错序
3.约束条件的流程：from -> where -> group by -> having -> distinct -> order by -> limit
4.字段可以起别名
5.字段可以直接做运算 select age + 1 'new_age' from emp;
6.分组后的条件均可以使用聚合函数
''''''
3. 伪代码解析流程
def from():return "查询的文件"
def where(file):return "条件筛选后的结果"
def group_by(res):return "分组后的结果"
def having(res):return "再次过滤后的结果"
def distinct(res):return "去重后的结果"
def order_by(res):return "排序后的结果"
def limit(res):return "限制条数后的结果"def select(from=from, where=null, ..., limit=null):file = from()res = where(file) if where else fileres = group_by(res) if group_by else res...res = limit(res) if limit else resreturn res
select(where=where, group_by=group_by)

1. 比较运算符```=    <    >    <=        >=        !=``````select * from emp where area!="上海";```2. 区间运算符between 10 and 20    # 表示10-20 之间in (10,20,30)            # 表示10,20或者30```select * from emp where id between 3 and 5; # [3, 5], 闭合区间,包含3和5
```3. 逻辑运算符and        or        not```select * from emp where area='山东' and port='济南';mysql> select * from emp where area='山东' and port='济南';+----+-------+--------+------+--------+--------+--------+-----------+| id | name  | gender | age  | salary | area   | port   | dep       |+----+-------+--------+------+--------+--------+--------+-----------+|  2 | owen  | 男     |   38 |    9.4 | 山东    | 济南   | 技术部     || 10 | kevin | 男     |   36 |    5.8 | 山东    | 济南   | 人资部     |+----+-------+--------+------+--------+--------+--------+-----------+2 rows in set (0.00 sec)
```4. 相似运算符(like)下划线或者%可以进行模糊匹配```select * from emp where name like '__en%'; # 匹配出Owenmysql> select * from emp where name like '__en%';+----+------+--------+------+--------+--------+--------+-----------+| id | name | gender | age  | salary | area   | port   | dep       |+----+------+--------+------+--------+--------+--------+-----------+|  7 | owen | 男     |   28 |    8.8 | 安徽    | 宣城    | 技术部    |+----+------+--------+------+--------+--------+--------+-----------+1 row in set (0.00 sec)
```5. 正则匹配(regexp)- 由于like完成模糊匹配的范围有限 , 可以模糊个数 , 但不能模糊类型- ------正则可以完成类型和个数的模糊匹配(只支持部分语法)
```pythonselect * from emp where name regexp '.*[0-9]+.*';  # 匹配出名字中带数字的记录mysql> select * from emp where name regexp '.*[0-9]+.*';+----+------+--------+------+--------+--------+--------+-----------+| id | name | gender | age  | salary | area   | port   | dep       |+----+------+--------+------+--------+--------+--------+-----------+| 13 | san1 | 男     |   30 |      6 | 上海    | 浦东   | 咨询部    || 14 | san2 | 男     |   30 |      6 | 上海    | 浦西   | 研发部    |+----+------+--------+------+--------+--------+--------+-----------+2 rows in set (0.00 sec)```

___聚合函数___max()：最大值
min()：最小值
avg()：平均值
sum()：和
count()：记数
group_concat()：组内字段拼接，用来查看组内其他字段

___example___

# eg1
# 每个部门的平均薪资
select dep, avg(salary) '平均薪资' from emp group by dep;

mysql> select dep, avg(salary) '平均薪资' from emp group by dep;
+-----------+-------------------+
|    dep    |    平均薪资        |
+-----------+-------------------+
|   咨询部   | 6.250000059604645 |
|   人资部   | 5.400000027247837 |
|   技术部   | 5.850000023841858 |
+-----------+-------------------+
3 rows in set (0.00 sec)

# eg:2
# 每个部门都有哪些人
select dep, group_concat(name) 'name' from emp group by dep;

mysql> select dep,group_concat(name) 'name' from emp group by dep;
+-----------+---------------------------------------+
|    dep    | name                                  |
+-----------+---------------------------------------+
|   咨询部   | san1,san,ying,zero                    |
|   技术部   | san2,kevin,owen,jiboy,tank,jerry,engo |
|   人资部   | monkey,yangsir                        |
+-----------+---------------------------------------+
3 rows in set (0.04 sec)

# eg3
# 各性别中附属于教学部的最高薪资
select max(salary) '最高薪资', gender from emp where dep='教学部' group by gender;

1.各部门的平均薪资
select dep, avg(salary) '平均薪资' from emp group by dep;2.平均薪资大于6w的部门(部门与部门的平均薪资)
解决: 以dep进行分组, 以avg(salary)作为判断条件(筛选)
select dep, avg(salary) '平均薪资' from emp group by dep having avg(salary) > 6;# 总结: having通过聚合函数结果完成筛选
select max(salary) from emp having max(salary) > 9.4;
# 虽然没有明确书写group by, 但在having中使用了聚合函数,所以该查询就将整个表当做一个默认大表来考虑,所以查询的字段只能为聚合函数的结果

# eg：order by age desc => 按照年龄降序
select * from emp order by age desc;# 需求：
# 将部门按照部门平均工资降序方式排序
select dep, avg(salary) from emp group by dep order by avg(salary) desc;

# limit 1：只能显示一行数据
# limit 6,5：从第6+1行开始显示5条数据(索引从0开始)select * from emp limit 1;
select * from emp limit 6,5;# 需求:
# 获得薪资最高的人的一条信息
select * from emp order by salary desc limit 1;